The present invention relates to a power supply device for an X-ray tube using a high-frequency inverter.
Various kinds of power supply devices for an X-ray tube using a high-frequency inverter of some kHz to tens kHz have been proposed, in recent years, in view of high-speed response characteristic and low ripple characteristic. Such power supply devices for an X-ray tube are disclosed. For example, in U.S. Pat. No. 4,720,844 issued Jan. 19, 1988 to J. Bougle, U.S. Pat. No. 4,400,822 issued Aug. 23, 1983 to Kuhnke et al., and U.S. Pat. No. 4,117,334 issued Sep. 26, 1978 to E. J. Strauts.
FIG. 3 shows a known power supply device for an X-ray tube of the type. This power supply device includes a direct current (DC) power supply 1 having an accumulator or a rectifier, a high-frequency inverter 2 for converting DC voltage of the DC power supply into high-frequency voltage, a high-tension transformer 3, and a high-tension bridge rectifier 4. Usually, the high-tension transformer 3 and the high-tension bridge rectifier 4 are received in a metallic high-tension tank 5 filled with insulation oil, and grounded.
The power supply device for an X-ray tube has positive and negative output terminals which are connected with an anode A and a cathode K of an X-ray tube 8 through two high-tension cables 6 and 7. The high-tension cables 6 and 7 include covering conductors 6a and 7a, respectively, which are grounded through a casing of the high-tension tank 5. A tube current detecting circuit 9 is arranged to determine a tube current of the X-ray tube 8 out of a current passing a midpoint between two secondary windings N2 and N'2 of the high-tension transformer 3. A filament power supply is further needed to operate the X-ray tube 8, however, a description thereof is omitted due to an absence of direct connection with the present invention.
The operation of the power supply device for an X-ray tube will be described. A high-frequency output voltage of the high-frequency inverter 2 is supplied to the high-tension transformer 3 at a primary winding N1 thereof. The two secondary windings N2 and N'2 of the high-tension transformer 3, which are the same in number of turns, are connected in series to assume the same polarity, and have a connecting point a which is a neutral point of an output of this power supply device, and grounded. The tube current detecting circuit 9 is insertedly arranged to the secondary winding N'2.
Assuming a voltage generated in each of the secondary windings N2 and N'2 to be E, the high-tension bridge rectifier 4 receives an alternating current (AC) input voltage of 2 E, and provides a rectified voltage of 2 E. Since the connecting point a of the secondary windings N2 and N'2 is grounded, the power supply device has a positive output voltage of +E, and a negative output voltage of -E. These voltages are applied to the X-ray tube 8 at the anode A and the cathode K thereof through the positive and negative high-tension cables 6 and 7.
For facilitating a high-tension insulation construction of the power supply device, the connecting point a of the secondary windings N2 and N'2 is grounded, and the anode voltage is set to +E while the cathode voltage is set to -E. With such power supply device for an X-ray tube of the high-frequency type, capacitances between core wires of the high-tension cables 6, 7 and the covering conductors 6a, 7a assume a filter effect, resulting in a decreased high-frequency ripple of a voltage to be applied between the anode A and the cathode K of the X-ray tube 8.
A problem faced in such known power supply device for an X-ray tube is that:
1--each of the capacitances of the high-tension cables 6 and 7 merely serves as a ripple reduction filter; and
2--since the tube current is detected at the series connecting point a of the secondary windings N2 and N'2 of the high-tension transformer 3, a charge/discharge current of a distributed earth capacitance of each of the secondary windings N2 and N'2 is detected simultaneously, resulting in a difficulty of accurately detecting the tube current as a frequency of the high-frequency inverter 2 increases.
It is, therefore, an object of the present invention to provide a power supply device for an X-ray tube having a decreased size, and a reduced manufacturing cost.
It is another aspect of the present invention to provide a power supply device for an X-ray tube which allows an accurate detection of a tube current of an X-ray tube.